When to Roam
Roaming is an integral part of playing mid lane. The best time to roam is after you shove a wave under the enemy's tower because that optimizes your gold that you get while making them choose between missing a wave to follow you or getting a wave and not able to counter gank. You can also go straight from base to top or bot, this catches a lot of people off guard. It's good practice to gank the same lane your junglers ganking to increase the odds of getting a kill out of the gank.
Why is roaming important?
As mid lane, you are in the best position to help yourself and other lanes since it's the middle of the map. You shouldn't always roam, but when you do it can get you, or your laners snowballing. It also helps when your losing lane. It can put you back into your lane if you get kills around the map, or you can put an ally laner further ahead than your lane opponent was.